SSDT (Site Selective Diversity Transmit) is one of the important technologies used in the advanced mobile communication systems, and a correct identification of the primary cell is an important step affecting the performance of SSDT technology.
Site Selective Diversity Transmit (SSDT) is a macro diversity method under the soft handoff. The operation procedure is as follows: a cell is selected by UE (user equipment) as a primary cell from an active set, and the other cells are non-primary cells. The main object is to perform down link transmit from the primary cell, and thereby, to suppress the interference produced by the multi-transmit in the soft handoff mode. The second object is to obtain fast site selection when the network does not take part in, thereby, to ensure the advantages of the soft handoff. In order to select a primary cell, a temporary identifier (ID) is assigned to each of the cells, and the ID of the primary cell is transmitted periodically by UE to the connected cells via the up link FBI (feedback indication) fields, and the cells selected by UE as non-primary cells will stop to transmit. The activation of SSDT, the end of SSDT, and the assignment of the IDs are performed via the high layer signaling.
An ideal cell identification situation supporting SSDT technology is indicated in FIG. 1. Wherein, the cell where base station 1 is located in is a real primary cell selected by UE. Base station 3 and base station 4 are non-primary cells located in the active set of UE. In FIG. 1, base station 1 is identified by itself as a primary cell, therefore, the down link user data are transmitted to UE; and the other base stations are identified correctly by themselves as non-primary cells, therefore, although the up link signals transmitted by UE are received, however, the down link user data are not transmitted to UE.
The condition for deciding a cell by itself whether it is a primary cell or a non-primary cell is stipulated in Rel′99 V3.7.0 of 3GPP Specification TS25.214, the stipulation is as follows:
The ID code word of the primary cell is transmitted periodically by UE via a part of the up link FBI (S field of FBI) assigned to SSDT. If the following conditions are satisfied with simultaneously, then a non-primary cell will be considered by itself:
A′. The received ID code word of the primary cell and the ID code word of itself are not matched;
B′. The quality of the received up link signals satisfies with a quality threshold Qth, the said threshold is a parameter defined by the network.
C′. The number of the bits of the ID code word drop caused by using the up link compression mode does not excess an upper limit, the said upper limit is (int) NID/3, wherein, NID is the length of the ID code word.
Otherwise, the cell itself will be considered as a primary cell by the cell.
It is indicated particularly that the conditions A, B, and C stated in the other portions in the patent are conditions for identifying directly the primary cell, and in order to distinguish from the condition for identifying the non-primary cell introduced in Background Art, the symbols A′, B′, and C′ are used herein to indicate the conditions for identifying non-primary cell.
It can be seen, under SSDT mode, the condition for a cell to decide itself being a non-primary cell is critical, and the condition to decide itself being a primary cell is relatively not critical, and the object for doing so is to avoid as much as possible the situation that none of the cells considers itself as a primary cell from occurring. Under SSDT mode of the soft handoff, once there is no primary cell, it will cause directly that no cell transmits the service data to UE, causing the data being lost and the quality of the transmission information being degraded.
It is necessary to explain particularly, wherein, the object of the condition B′ is to make that only the cell, which identifies reliably the ID code word of the said cell being not matched with the received ID coed word, considers itself as a non-primary cell. If that the quality of the up link signals is not good enough to ensure the reliability of the result of the match algorithm is discovered by the cells, then the cells will consider themselves as the primary cell under such situation regardless the match result.
It is considered by the prior primary cell identification technology that if the cell conforms with the above three conditions A′, B′, and C′ simultaneously, then the cell will identify itself as a non-primary cell. Wherein, the function of condition B′ is to ensure the reliability of a cell deciding itself as a non-primary cell. However, this will cause the following improper situation to occur: If the quality of the up link signals received by a cell does not satisfy with the threshold Qth, that is, when the up link signals are under a bad situation, the said cell will certainly decide itself as a primary cell. In practical, if the difference of the propagation distance between the respective cells and UE is considered as an important affecting factor of the quality difference of the up links and down links between UE and the respective cells in the active set, then the possibility of the said cell being selected by UE as a primary cell will be relatively small.
The above situation corresponds to that under SSDT mode, except the real primary cells in the active set, some cells with rather bad quality are also identified as the primary cells, while some cells in the active set with better signal quality are decided as the non-primary cells. This causes some bad results, because the cells with the quality that is bad to a certain extent (below Qth) will transmit DPDCH (dedicated physical data channel) to UE, and the said signal will interfere significantly the other users. While the DPDCH signal of the said cell provides rather small function to UE, and it is difficult to indicate the benefit of SSDT. On the other hand, under such situation, UE may combine DPDCH signals of the real primary cells and the cells with bad quality, (note: the bad cells may make no contribution to the combination). Corresponding to the situation that UE may combine a plurality of cells in active set under the condition without SSDT, the receiving performance under SSDT will be worse. Therefore, under the above situation, not only does not present the benefit of reducing the interference under SSDT, but also it is even worse than not introducing SSDT.
This situation of the prior primary cell identification technology is explained in FIG. 2. The base station 1 is identified correctly as a primary cell because of not conforming with the condition A′; the base station 2 conforms simultaneously with three conditions A′, B′, and C′ for identifying the non-primary cells, therefore, it identifies correctly itself as the non-primary cell; and the base station 3 and base station 4 do not conform with the condition B, that is, the quality of the up link signals is worse than the threshold Qth, therefore, they decide themselves as the primary cells, and transmits to UE the down link service data. It can be seen, with respect to the base station 3 and base station 4, the signals with better quality may be provided by the base station 2, however, not transmitted; the signals transmitted by the base station 3 and base station 4 produces the additional down link interference.